One of the most important issues currently with wireless communication systems is how to increase the capacity of the wireless communication system. One of the new areas being explored is the use of directional antennas to improve the link margin of the forward and reverse links between base stations and wireless transmit/receive units (WTRUs). The increased gain of the directional antenna over the typical omni-directional antenna provides an increased received signal gain at the WTRU and the base station.
A passive-antenna array, such as shown in the three-dimensional view of a prior art smart antenna 100 of FIG. 1, has been developed as an efficient and low cost smart antenna for Subscriber Based Smart Antenna (SBSA). The smart antenna 100 comprises one active element 102 disposed in the center top portion of a ground plane 106 and three passive elements 104 surrounding the active element 102. Each passive element 104 comprises an upper half 104a and a lower half 104b. The upper halves 104a of the passive elements 104 are connected to the ground plane 106 through reactive loads 112, respectively. The reactive loads 112 are variable reactance, which is changeable from capacitive to inductive by using varactors, transmission lines or switching. By varying the reactive loads 112, the radiation pattern can be changed. The lower halves 104b of the passive elements 104 are directly connected, (i.e., shorted), to the ground plane 106. Since the lower half 104b is shorted, the beam is tilted upward, which degrades the capability of steering a beam in elevation. The smart antenna 100 is capable of forming and steering a beam only in azimuth, not in elevation. With the need of enhanced capacity of a wireless communication system, more refined use of smart antennas requires the beam to be steered in both azimuth and elevation.
FIG. 2 is a diagram of another prior art smart antenna 200. The smart antenna 200 has a similar configuration as the smart antenna 100. However, the difference is the number of passive elements 204. The smart antenna 200 comprises one active element 202 and two passive elements 204. The upper halves 204a of the passive elements 204 are connected to the ground plane 206 through variable reactances 212, but the lower halves 204b are shorted to the ground plane 206. Since the lower halves 204b of the passive elements 204 are shorted to the ground plane 206, the beam is tilted upward, which degrades the capability of steering a beam in elevation.
Edge impedance of the ground plane is also a cause of beam tilt. Many antennas are built on a finite ground plane, which has the advantage of providing an easy interface with, and good isolation from, the remainder of the wireless communication system. However, beam tilt is inevitable because the edges of the ground plane operate as a radiation scatterer. The ground plane absorbs and re-radiates the radio wave and the re-radiated radio wave interferes with the antennas' direct radiation, thereby resulting in a tilted beam.
The ground plane is finite with respect to the wavelength of transmitted and received signals. This is especially true when the smart antenna is implemented in a WTRU, where the overall size of the antenna is restricted. Because of the interaction between the small ground plane and the antenna element, the beam is tilted upward. Accordingly, the strength of the beam along the horizon is decreased.
In steering a beam both in azimuth and elevation, it is desirable to vary the beam width of an antenna in elevation. Fixed elevation beam width antennas can cover a fixed elevation sector. Some locations may require a larger coverage in elevation, but some locations may require a smaller coverage in elevation. Generally, a narrower beam can provide more gain and larger information capacity. Therefore, there is a need for adjusting the beam width in elevation.